1. Field of the Invention
The present invention relates to an image forming apparatus (e.g., a color laser printer, a color copy machine, or a color facsimile) that performs an image forming operation including electrophotographic processes. In particular, the present invention relates to a technique which can reduce unnecessary radiations that may be emitted from the image forming apparatus.
2. Description of the Related Art
An electrophotographic color image forming apparatus forms a latent image by causing a light irradiation unit to irradiate a charged photosensitive member with light (i.e., perform an exposure operation), and forms a toner image on the photosensitive member by causing a developing device to adhere toner particles to the latent image on the photosensitive member.
Usually, the light irradiation unit irradiates a limited area of the photosensitive member (i.e., a toner adherent area) with light. However, as discussed in Japanese Patent Application Laid-Open No. 2012-58721, for the purpose of maintaining an electrical potential of a toner non-adherent area of the photosensitive member at an appropriate level to suppress the generation of a faulty image, it is conventionally known that the light irradiation unit irradiates the toner non-adherent area of the photosensitive member with a very small quantity of light in such a way as to prevent toner particles from adhering to the photosensitive member. The above-described minute exposure for the toner non-adherent area is generally referred to as “background exposure.”
To express the density of an image, the light-emission time of a light source of the light irradiation unit can be changed for each pixel. For example, performing a pulse width modulation control is effective to change the exposure amount because the time interval of drive current flowing in the light source is adequately adjustable. To perform background exposure processing based on the above-mentioned pulse width modulation applied to the drive current, the drive current flows during a minute time period by an amount required to obtain a very small quantity of light.
FIG. 19 illustrates image data and a video signal in a case where pixels to be subjected to the background exposure processing are continuously arrayed. As illustrated in FIG. 19, minute time period light emission is performed for each pixel, in which the minute time period corresponds to a very small quantity of light less than one pixel.
When the pixels to be subjected to the background exposure processing are continuously arrayed, drive current repeating at a constant interval and having a minute time period flows in the light source. In this case, a significant amount of current flows across a driving circuit of the light source or a cable supplying current to a power source line thereof. An inductance component of the power source line induces a high-frequency noise voltage.
Then, high-frequency noises included in the noise voltage induce resonance in the power source line cable. The power source line cable serves as an antenna, which can spatially emit a part of electromagnetic energy of the high-frequency noise as electromagnetic waves. The electromagnetic waves emitted in this manner are referred to as unnecessary radiations (noises).
FIG. 20A illustrates image data to be continuously subjected to the background exposure processing. FIG. 20B illustrates unnecessary radiations generated when the light source emits light based on the image data illustrated in FIG. 20A. The unnecessary radiations tend to occur greatly at specific frequencies relevant to the light-emission period of the light source.
According to the technique discussed in Japanese Patent Application Laid-Open No. 2012-58721, an image generation unit is configured to generate a background exposure oriented clock signal to perform a minute light-emission operation for the background exposure processing in addition to an image forming exposure oriented clock signal. Further, background exposure oriented clock control frequencies are decentralized within a predetermined frequency range so that the background exposure oriented minute light emission can be prevented from being periodical and the generation of unnecessary radiations can be suppressed.
However, according to the configuration discussed in Japanese Patent Application Laid-Open No. 2012-58721, it will be required to newly provide a background exposure oriented clock generation circuit to reduce unnecessary radiations, in addition to an image forming exposure oriented clock generation circuit. Therefore, the costs will increase by an amount corresponding to the newly added clock generation circuit.